A MIME::Decoder subclass for the "base64" encoding.
The name was chosen to jibe with the pre-existing MIME::Base64 utility package,
which this class actually uses to translate each chunk.

When decoding,
the input is read one line at a time.
The input accumulates in an internal buffer,
which is decoded in multiple-of-4-sized chunks (plus a possible "leftover" input chunk,
of course).

When encoding,
the input is read 45 bytes at a time: this ensures that the output lines are not too long.
We chose 45 since it is a multiple of 3 and produces lines under 76 characters,
as RFC 2045 specifies: The encoded output stream must be represented in lines of no more than 76 characters each.